Unraveling the fabric of space-time with Dr. Ian O'Neill

Could Extraterrestrial Genes Be Like Ours?

This is probably one of the biggest questions that hang over science fiction story lines: Will extraterrestrials have any resemblance to Life As We Know It™? To be honest, to toy with the thought of anything other than carbon-based life is pure conjecture, just because there might be some other form of life (such as silicon-based creatures), doesn’t mean there is (doesn’t mean there isn’t, either). So, here we are with the only form of life we know and understand, carbon-based life that was somehow spawned via a crazy mix of amino acids and some astronomical or terrestrial event that sparked the formation of prokaryotes (a.k.a. the simplest single-celled speck of life) some 4 billion years ago.

So we have an understanding of what formed life on Earth, perhaps if we look for the traces of evidence that evolved into Life As We Know It™ we can gauge whether extraterrestrial life has-formed/is-forming/will-form elsewhere in the observable Universe. From simulations of Earth evolution, scientists have predicted that 10 types of amino acids should form with the planet. These 10 amino acids are found inside the proteins of all living things on Earth. The same 10 amino acids have been found inside meteorites. Therefore, we already have a connection with the amino acids we find here on Earth and amino acids found in chunks of rock from elsewhere in the Solar System.

Now, a group of Canadian researchers have found that the same 10 amino acids are readily available elsewhere in the cosmos. Does this mean the components for life are common, not only on Earth, in the Solar System, but also in the Milky Way (and beyond)? It looks like it…

Not being a biologist, I took it upon myself to try to familiarize myself with the astounding claims that are being made in this research. In a nutshell, proteins contain 20 types of amino acids. We don’t really know why there are 20, but there are. 10 of those amino acids have been predicted to form during the planetary evolution of Earth. Whether these chemicals were formed in-situ or whether they were carried here via meteorite or comet, it is unknown, but the point is that they were here 4 billion years ago. During this time, these acids combined to create a prebiotic slush, the forerunner to the most basic single-celled bacteria, prokaryotes. Somewhere along the line before the precursor to all life formed, it is thought that the remaining 10 complex amino acids found in modern-day proteins were formed by some kind of secondary process. But the main point is, the first 10 amino acids were here, on Earth, 4 billion years ago.

During early simulations of the Earth’s atmosphere after it was formed from the proto-planetary disk (from Miller’s atmospheric discharge experiments starting in 1953), got battered by the Late Heavy Bombardment and matured its atmosphere, scientists could account for 10 of the 20 amino acids currently found in life. Although this was interesting, it wasn’t until the chemicals found inside meteorites were analysed that a connection was made. The exact same amino acids were present inside pieces of rock that originated from elsewhere in the Solar System. Suddenly, we had a connection between the amino acid chemistry on Earth and amino acid chemistry in other Solar System bodies.

Awesome. But it gets better.

Paul Higgs and Ralph Pudritz, researchers at McMaster University in Hamilton, Canada, have expanded the scope of this amino acid hunt. They have investigated the thermodynamic situations where these basic amino acids form, and according to their calculations, the genetic code of possible extraterrestrial life could evolve to utilize these amino acids in the same way as terrestrial life. The thing is, Higgs and Pudritz have ranked the order at which these amino acids are likely to occur in proteins, and you guessed it, the abundances of the 10 amino acids exactly match that of the 10 amino acids predicted in early atmospheric models of the Earth and the abundances found in meteorite samples.

In short, if there’s an Earth-like exoplanet out there, there is a very high probability that the same abundances of amino acids will be there too. The thermodynamic amino acid arguments as used by Higgs and Pudritz can be applied to everything from the atmosphere of a temperate exoplanet, or interstellar gas clouds. We already have evidence that amino acids exist in nebulae, and as KFC on the arXiv physics blog says, “What’s the betting that these amino acids are the same as the prebiotic 10 that Higgs and Pudritz finger?”

Indeed, it appears that not only does Mother Nature favour this blend of amino acids, but it appears to be common throughout the cosmos. But what does this mean? I’ll leave that to the Higgs and Pudritz to explain, in what could be one of the most important conclusions ever to be printed on a PDF:

In summary, we have shown that in spite of the conflicting opinions on the mechanisms and locations of molecular synthesis, thermodynamics cuts across these distinctions and predicts which amino acids are formed most easily. Our results also indicate that a certain degree of universality would be expected in the types of organic molecules seen on other earth-like planets. Should life exist elsewhere, it would not be surprising if it used at least some of the same amino acids we do. Simple sugars, lipids and nucleobases might also be shared. Our analysis suggests that the first genetic code was a stripped down version of our present code, one whose simpler structure reflected the limited set of available amino acids in the prebiotic environment. Although there are countless ways that the code could have developed from those origins, the combined actions of thermodynamics and subsequent natural selection suggest that the genetic code we observe on the Earth today may have significant features in common with life throughout the cosmos. — “A thermodynamic basis for prebiotic amino acid synthesis and the nature of the first genetic code,” Paul G. Higgs and Ralph E. Pudritz, 2009

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3 responses to “Could Extraterrestrial Genes Be Like Ours?”

Excellent article, and I downloaded the paper, too. Thanks for posting this.
Of course, I HAVE to say it’s an excellent article… because its subject has been what I’ve been reckoning for quite a while now:
1) that life as we know it is abundant throughout the Universe – it’ll probably form as early as possible wherever it can and hang on for as long as the conditions allow it to hang on.
2) given that the same science applies throughout (this) Universe, the same thermodynamics, which ensure that the “star dust” is the same everywhere, and which also control the preferred way of how molecules and atoms combine…
…so, in the end, for me, it stands to reason that ET should not be very different from us, and probably be bothered by similar athlete’s foot fungi and mosquitoes, and eat things we may recognise as something like bananas and potatoes and peanuts….

Of course, there could also bee life as we don’t know it, even on Planet Earth – but we haven’t defined it, so we cannot detect it.

“genes … like ours”. Well, that is a very speculative possibility of course.

First off, for the genes to look somewhat like ours they need to code for the same protein families, which then AFAIU canalizes much of later evolution. But there isn’t any guarantee for that the first functional proteins would look anything like ours, since function, and the order it evolved in, is contingent.

Second, the paper depends on the coevolutionary theory for the code. But there are AFAIU several theories today, and currently no test for which is correct.

Third, the paper gloss over that the concentrations of prebiotic amino acids (AAs) are AFAIU nowhere near enough to contribute to abiogenesis. If the metabolic pathways decide the early AAs, it is anybody’s guess which ones will be used.

Albeit it is interesting and probably not unrelated that code-grouped AAs are the same set that ranks naturally. Maybe there is a canalization of pathways as well.

I guess I would have been much more happy if the authors had suggested any means of testing their interesting albeit putative hypotheses. Perhaps the above correlation could be put to use for that.

A nitpick: In the same way that there are AFAIU many later modified codes, especially in different mitochondrias, there are more than 20 AAs by late modifications.

I’m not a biologist, but I’m currently reading a paper from 2002 by Cavalier-Smith that mentions “selenocysteine, the twenty-second amino acid … which is encoded by UGA, which usually signifies termination.” Seems these AAs are inserted co-translationally (at least in the eukaryotes he is describing there), in this case recognized by specific molecules by some extra code after the UGA.